Low-coherence optical microscopy or optical coherence microscopy uses light with short coherence length. The well-known case is: “white-light interferometry”, which became recently more known as: “optical coherence tomography”. However, when lenses and microscope objectives are used to create interferometric images, in what is known classically as “interference microscopy” or today as “full-field optical coherence tomography” the spatial coherence starts to play a critical role. In this article the coherence effects in low-coherence optical microscopy are reviewed. As this technology is becoming increasingly publicized due to its importance in three-dimensional imaging, particularly of scattering biological media and optical metrology, the understanding of the fundamental physics behind it is essential. The interplay between longitudinal spatial coherence and temporal coherence and the effects associated with them are discussed in detail particularly when high numerical apertures are used. An important conclusion of this study is that a high-contrast, high-resolution system for imaging of multilayered samples is the one that uses narrowband illumination and high-NA objectives with an index-matching fluid. Such a system, when combined with frequency-domain operation, can reveal nearly real-time three-dimensional images, and is thus competitive with confocal microscopy.